This application claims the benefit of Japanese Patent Application No. 2000-101738 which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an eyepiece lens system which is used in an observation optical system for a binocular, a telescope, or a microscope, as well as to a binocular, a telescope, and a microscope employing such an eyepiece lens system.
2. Related Background Art
In case of an eyepiece lens to be used in an observation optical system, it is required to provide an eye relief having the sufficient length in order to easily observe the entire field of view. However, in case of a general eyepiece lens system, there is a tendency that the shorter the focal length of the lens system is, the larger a sum of the refractive power of the lens surfaces for constituting the optical system. As a result, aberrations typically including a curvature of field are deteriorated. Also, in case of the general eyepiece lens system, the eye relief can be often obtained in the size of only 80% or around of the focal length. As a result, it is impossible to obtain a sufficient eye relief in an eyepiece lens having a small focal length.
Furthermore, in case of the general eyepiece lens, when an apparent field of view is enlarged while maintaining excellent aberrations even up to the circumference of the field of view, the eye relief becomes short. On the other hand, when the eye relief is lengthened while maintaining a fixed apparent field of view, the diameter of the eyepiece lens is increased, thereby deteriorating the aberrations on the circumference of the field of view.
A distortion especially has a tendency to increase in proportion to the size of the apparent field of view. Thus, it is very difficult to enlarge the apparent field of view and the eye relief at the same time since it is difficult to correct this distortion satisfactorily by the use of a lens system which is generally formed only by spherical surfaces.
Consequently, in order to solve these problems mentioned above, there have been proposed a technology which employs an aspherical surface in an eyepiece lens system.
As a technology for forming an aspherical form on a lens surface, there has been proposed fine grinding, or polishing, or a use of an aspherical mold lens.
However, fine grinding and polishing take a lot of time for forming the aspherical form, which may easily result in increase in the processing cost. Also, when a mold lens is to be produced, there arise a large number of restrictions on the material, the form, and the like of the lens by these methods, which undesirably lower the degree of freedom in the optical design.
Accordingly, an object of the present invention is to provide an eyepiece lens system which employs an aspherical lens having a few restrictions on optical designing to be formed at a low cost, in which an eye relief is long enough to be equivalent to 80% or more of the focal length of the eyepiece lens system with the aberrations, particularly a distortion, excellently corrected even up to the circumference of the field of view.
Another object of the present invention is to provide a telescope, a binocular, and a microscope capable of observing an object to be observed as an excellent image by using such an eyepiece lens system as described above.
In order to achieve the above objects, according to the present invention, there is provided an eyepiece lens system which comprises a cemented lens and a positive lens, and in which at least one of the lens surfaces is formed aspherical, and this aspherical surface is of a hybrid type formed by applying a synthetic resin layer on a basic spherical surface made of glass.
Also, according to the present invention, there are provided a telescope, a binocular as well as a microscope employing such an eyepiece lens system as described above.